1. Field of the Invention
This invention relates to a thyristor having an amplifying gate structure and more particularly to a thyristor having a large di/dt capability (current rise rate capability) at the transient period of turn-on.
2. Description of the Prior Art
When a forward voltage is applied between the anode and the cathode electrode of a thyristor and a triggering signal voltage is applied between the gate and the cathode electrodes of the thyristor, the resulting current (gate current) flowing between the gate and the cathode electrode causes the thyristor, which was cut off until then in the forward direction, to turn conductive. The shift of the thyristor from cut-off to conductive state is referred to as the turn-on of the thyristor. The turn-on of the thyristor takes place in the following manner. Namely, the resulting gate current first turns on a small area of the thyristor which is nearest to the gate electrode and on the periphery of the cathode electrode and then the thus generated conductive area expands throughout the whole body of the thyristor with the lapse of time.
Accordingly, if the rate of rise of the forward current, i.e. di/dt, is large, a large forward current flows through a small conductive region. As a result, the current density becomes so high that the power loss in the conductive region becomes considerable. In some extreme cases, it may happen that the thyristor is destroyed by the conduction current having so high a density. In order to prevent such a fault, therefore, various gate structures have been proposed. One of the typical examples of such gate structures is an amplifying gate structure.
FIG. 1 shows an example of the amplifying gate structure and FIG. 2 shows the equivalent circuit of the exemplary structure in FIG. 1. When a trigger signal is applied between a gate electrode 4 and a cathode electrode 2, with a forward voltage applied between the cathode electrode 2 and an anode electrode 3, gate current i.sub.G flows through a path: the gate electrode 4--p base 9--auxiliary n emitter 12--auxiliary cathode electrode 5--p base 9--main n emitter 8--the cathode electrode 2. Accordingly, an auxiliary region 14 first turns on to cause an auxiliary current i.sub.a to flow. The auxiliary current causes a thyristor region 15 having a large area under the auxiliary cathode electrode 5, to turn on so that a main current i.sub.b flows. The forward current in the early stage of the turn-on period is therefore divided into two components; one flowing through the auxiliary thyristor region and the other through the main thyristor region. Namely, the switching energy is shared to the two regions, whereby the di/dt capability in the early stage of the turn-on period can be made rather large.
Thus, the amplifying gate structure has proved to relatively improve the di/dt capability. However, even in such a thyristor as having an amplifying gate structure, electrical and/or thermal injury and/or breakdown in the auxiliary thyristor or in the vicinity thereof are liable to take place to meet the recently increasing requirements for withstanding high voltages, treating large currents and operating at high speeds. Hence, it is difficult to attain a sufficient di/dt capability. The reason for this is as follows.
The di/dt capability of a thyristor having an amplifying gate structure is determined depending on the larger one of the switching energies consumed per unit area of the conducting region of the auxiliary thyristor 14 and per unit area of the conducting region of the main thyristor 15, in the early stage of the conducting period.
Accordingly, if the resistance value of the resistor R.sub.G shown in FIG. 2 is small, a sufficient gate current is supplied for the main thyristor 15 from the auxiliary thyristor 14. Therefore, by making the opposite portions of the auxiliary cathode electrode 5 and the main n emitter 8 sufficiently long, it is probable that the initial conduction area will be easily increased and that the temperature rise will also be limited. In the case of the auxiliary thyristor, however, the lengths of the parallelling portions of the gate electrode 4 and the auxiliary n emitter 12 are made large, a large gate current is needed. For a small gate current remarkably causes uneven starts of turn-on periods and local turn-ons in the auxiliary thyristor 14 so that the initial conduction area cannot be increased. Also, since the auxiliary current i.sub.a continues to flow through the auxiliary thyristor 14 even after the main thyristor 15 has been turned on, the increase in the di/dt of the main current necessarily causes the increase in the auxiliary current. This is a drawback that the switching energy consumed in the auxiliary thyristor 14 further increases. On the other hand, if the resistance of the resistor R.sub.G in FIG. 2 is made large, the current through the auxiliary thyristor 14 can be decreased to a certain extent. In this case, however, the heat generated by the transverse resistance R.sub.G becomes appreciable and also the gate current injected into the gate of the main thyristor 15 becomes small so that the turn-on of the thyristor 15 is delayed. And since most of the auxiliary current i.sub.a flows through the auxiliary thyristor 14 until the main thyristor 15 has been turned on, the switching energy consumed by the auxiliary thyristor 14 cannot be increased.
As apparent from the above description, even with a thyristor having a conventional amplifying gate structure, if the rate di/dt of rise of the anode current (i.sub.a -i.sub.b) is high, electrical and/or thermal injury and/or breakdown will take place in the auxiliary thyristor or in the vicinity thereof.
In order to prevent such electrical and/or thermal damage to the auxiliary thyristor, some measures have been proposed. For example, U.S. Pat. No. 3,526,815 (issued to P. Svedberg et al) discloses the insertion of a resistor between the main and the auxiliary thyristor (FIG. 6) and the Japanese Patent Appln. Laid-Open No. 135478/76 teaches the insertion of an inductance between the main and the auxiliary thyristor (FIG. 2). These and other methods hitherto proposed, however, seem still unsatisfactory.